1. Field of the Invention
This invention relates generally to wireless digital communication systems. In particular, the invention discloses a technique for increasing the performance of a wireless communication link operating in the presence of an external source of intermittent interference.
2. Background
Cordless telephones have become increasingly popular in recent years. As improved technology is incorporated into cordless telephone designs, their sound quality and reliability have greatly improved, leading to the increasing proliferation and acceptance of cordless telephone devices in residential, and even commercial, environments.
However, as cordless telephones (which operate via a wireless radio frequency (RF) communication link established between the phone handset and a base unit) and other wireless devices have become more popular, the electromagnetic spectrum over which such devices communicate has, in turn, become increasingly crowded. Additionally, increasing numbers of other electronic devices are being used throughout society. Many such devices radiate electromagnetic energy that “pollutes” the spectrum over which wireless devices must communicate. As a result of these spurious transmissions and radiated noise, wireless devices commonly experience crosstalk and interference that inhibits the accurate and reliable transmission of signals.
In order to reduce the crowding of the electromagnetic spectrum designated for use by cordless telephones and other personal wireless devices, numerous frequency domain interference avoidance techniques, such as dynamic channel allocation, have been developed. Such techniques typically involve altering of the radio frequencies corresponding to the “channels” over which a wireless device communicates in an attempt to avoid externally generated radiated electromagnetic energy.
In devices that use dynamic channel allocation technique interference is detected by measuring quality criteria such as Received Signal Strength Indication “RSSI”, packet, bit and synchronization errors. For a given channel, if RSSI, a number of synchronization, packet and/or bit loss errors are more then some specified threshold values, the channel will be qualified as bad. An algorithm implemented in the device will choose a new replacement channel and information about replacement of the channel will be communicated between link ends.
Also, additional frequency bands have been allocated for personal wireless communications devices, thereby spreading out device traffic and reducing the potential for interference.
Systems known in the art utilize many techniques to avoid the effects of RF interference that occur in a device's communication band. Many such techniques involve changing the system's frequency domain RF transmission characteristics. For example, calls may be handed off to a different carrier frequency, or noisy channels in the hopping pattern of a frequency hopping spread spectrum system may be substituted for different channels. Such channel substitution techniques are particularly effective in avoiding continuous narrowband sources of interference, such as may be generated by other cordless telephones.
A simple known multiple access system uses non-overlapping frequency bands for different communication links. Signals from two transceivers can be simultaneously transmitted without interfering with each other as presented in FIG. 7. User1 70 will occupy channel F1 71 spaced apart by using Guard Band 72 from channel F2 73 that is occupied by User2 74. Several users, User1 70, User2 74, User 3 75 and User4 76 utilize a portion of frequency spectrum at the same time. In this method multiple access capabilities are achieved in the frequency domain and method is called Frequency Division Multiple Access “FDMA”.
Similarly, instead of using different channels that are assigned to different users, signals can be transmitted at different time slots T1 80, T2 81, T3 82 and T4 83 in a round-robin fashion, as shown on FIG. 8. Signals from two transceivers will occupy same channel F 84 but will be separated based on time of arrival. Multiple access is achieved in the time domain, and the method is referred to as Time Division Multiple Access “TDMA”.
The 2.4 GHz ISM band has become popular for use by cordless telephones and other wireless devices. Telephones operating in the 2.4 GHz ISM band commonly employ a communication protocol that is based on the Digital Enhanced Cordless Telecommunications (“DECT”) standard, as defined in European Telecommunications Standards Institute (ETSI) standard ETS 300 175, September 1996 edition. Due to the characteristics and regulation of the 2.4 GHz band, devices operating thereon may offer increased range and/or bandwidth compared to other available frequency bands.
The DECT standard is being widely adopted throughout the world for wireless applications including cordless telephones, wireless offices and wireless telephone lines to the home. The DECT standard allows for multiple communication links, multiple accesses, between devices on a single RF carrier frequency through the use of time domain multiplexing. Following the success of DECT in Europe, Africa and South America, a variant of DECT has been developed for the North American market called Worldwide Digital Cordless Telecommunications (“WDCT”). The WDCT standard is currently becoming popular for use at the 2.4 GHz ISM band.
DECT utilizes a TDMA variant called Multi Carrier, Time Division Multiple Access, Time Division Duplex “MC/TDMA/TDD”, as presented in FIG. 10. Ten channels are allocated from 1.880 GHz to 1.900 GHz. In the time domain, spectrum is subdivided into 10 ms wide TDMA frames 101 and 162. Each frame is divided in 24 sub-frames called “timeslots”. Timeslots 103 and 104, which are spaced apart by 5 ms, can form one full duplex communication link, called a “bearer”. The bearer shown has a downlink 105 that is associated with time slot 103, where the downlink direction is from a fixed part “FP” to a portable part “PP”. A link direction from PP to FP that is associated with time slot 104, is called an uplink. The first 12 timeslots in one TDMA frame are used for downlink 105, while the rest of the slots are used for uplink 106.
There are three types of bearers used in standard DECT system: Dummy, Traffic and Connectionless bearer. “Dummy” 107 bearer is established in downlink direction to all PPs, and is used to keep all PPs synchronized with FP and to support continuous broadcast of signalling information to all users all the time. “Traffic” 108 bearer is a full duplex bearer between a PP and an FP. A primary use for a traffic bearer is to support voice or other user related information exchange between two link parties. Typically, in the DECT system, two bearers are established between PP and FP link parties that communicate by voice: a Dummy and a Traffic bearer. Each of the two bearers can be on different channels.
Yet another commonly used access method is presented in FIG. 9, known as Code Division Multiple Access “CDMA”. In the example shown, different users can be separated in time and frequency domains using a particular version known as Frequency Hopping CDMA “FH-CDMA”.
Combining FH-CDMA with TDMA creates a more cost-effective access method, called Frequency Hopping TDMA “FH-TDMA”, as presented in FIG. 11. An example of such an FH-TDMA system can be found in current VTECH® 2.4 GHz and 5.8 GHz cordless telephones based on WDCT, FH-TDMA/TDD. In frame n−1 117, Traffic bearer 110 is established over downlink 113 and uplink 114 time slots at channel F1 115. Frequency hopping in this system is done on a “frame-by-frame” basis, performing a channel change for the traffic bearer in each frame. In frame n 118 the same Traffic bearer 110 is established in the same downlink 113 and uplink 114 time slots, over a different channel F2 116. In every other frame the channel changes. According to FCC regulations, for one traffic bearer, frequency hopping is performed from a sequence of 75 pseudo-randomly selected channels from a pool of 95 channels. Instead of frame-by-frame frequency hopping, in other known systems half-frame-by-half-frame frequency hopping is used, with change in channels at the half frame boundary.
However, one substantial difficulty faced by designers of electronic equipment utilizing the 2.4 GHz ISM band is interference generated by operation of a common household microwave oven. During their operation, microwave ovens generate substantial levels of RF energy throughout the 2.4 GHz frequency range. Therefore, when a microwave oven is in operation, a nearby, active 2.4 GHz cordless telephone of prior art design would commonly experience substantial interference. Such interference would degrade the sound quality of the telephone call to an objectionable, if not unusable, level. The impact of microwave oven radiation on cordless telephone operation is particularly significant when users place the cordless telephone base unit directly on top of a microwave oven. In certain circumstances, it is possible that a cordless telephone call conducted in the presence of microwave oven generated interference would be dropped altogether.
Prior art frequency domain interference avoidance techniques are of limited effectiveness in the presence of an interference source that emits interfering energy over a substantial portion of a communication band. It is estimated that a microwave oven may radiate substantial levels of interfering RF energy on a majority of the frequency channels defined in the 2.4 GHz ISM band.
Accordingly, it is an object of this invention to provide a method that can be used in conjunction with a wireless communications system to avoid electromagnetic interference radiated by a microwave oven, or similar source.
Another technical problem that this invention addresses is a problem of multi-path fading, superposition of the reflected signals at the receive side of the link. This problem impacts audio quality especially in frequency hopping systems, since for one communication connection a plurality of channels is used, all having different wavelengths. Therefore, a received signal is often faded and corrupted, resulting in a voice mute or unpleasant tone being received. This is more noticeable when amplitudes of the reflected and direct signals are similar, which is typically the case when link parties are further away, close to the end of an operational range.
Prior art frequency domain interference avoidance techniques can help reduce this problem by switching bad channels. However, audio corruption is still a problem, since there is time delay between when a channel is detected as bad until it is physically replaced by another channel. Until successful replacement of the bad channel, a user will experience a degradation in sound quality. Since the user can move together with a handset while continuing a conversation, an environment and position of reflected objects can change, creating paths with different lengths. Consequently, dynamic changing of channels may not be sufficient to provide a highest voice quality. Another disadvantage of using dynamic change of channels to avoid multi-path fading is in increased processing power and signaling throughput required for processing of channels quality and associated communication link commands.
Some prior art systems use antenna diversity in order to solve this problem. A principle of operation is based on a difference in physical paths between two antennas. A receive event will start by using a first antenna. During that time, a quality criteria can be measured such as receiver signal strength “RSSI” and/or ability of the receiver to synchronize on a preamble, alternating a one and zero pattern at the beginning of the transmission. After a specified period the receive event will be continued by using a second antenna, and the same quality measurement as used for the first antenna is performed. At some specified point in time, comparison of two quality measurements is performed, and a receiver is switched to use the antenna that exhibits better measurement results, so that the better antenna can be used to receive the rest of the data associated with the time slot.
A problem with the above method is that estimation of the quality criteria is performed over a short period of time that may not accurately represent channel quality for the duration of the whole time slot.
It is an object of this invention to provide a method that can be used in conjunction with a wireless communications system to avoid interference radiated as a result of multi-path fading.
Yet another technical problem is avoidance of interference produced by other nearby frequency hopping wireless systems. Known frequency hopping wireless systems often transmit over one channel for a short period, typically insufficient to trigger frequency domain interference avoidance. Since a first frequency hopping wireless system can corrupt different channels often used in a hopping sequence of a second system, voice quality of the second system can be degraded.
It is an object of this invention to provide a method that can be used in conjunction with a wireless communications system to avoid radiated interference from other wireless systems that utilize frequency hopping.
In addition to reliability and sound quality, power efficiency of a cordless telephone handset is an important consideration in cordless telephone design. Cordless telephone customers demand telephones with extended battery life, such that talk time and time between required charging of the telephone handset needs to be as great as possible. However, consumers also desire compact and light weight portable telephone handsets, which, in turn, limits the physical size and, in turn, the electrical capacity of the battery that may be incorporated. While compact, high energy density battery technologies are one solution, they tend to be expensive, thereby increasing the cost of a cordless telephone that uses high density batteries to extend talk time. Therefore, it is highly desirable, and therefore it is an object of this invention, to provide a cordless telephone design that is power efficient.
Finally, some advanced wireless communications systems utilize multiple communication links over a single time domain multiplexed data frame. For example, advanced cordless telephone base units may support multiple portable handsets; wireless data communications may involve multiple devices on different time slots of a common carrier; and Wireless Local Loop technology may provide wireless telephone line service to a plurality of handsets in one or more homes using a common RF carrier. Therefore, it is an object of this invention to provide an interference avoidance technique that efficiently utilizes the capacity of a communications channel.
These and other objects of this invention will become apparent to those of ordinary skill in the art in view of the invention described herein.